1. Field of the Invention
The present invention relates to wireless communication systems, and more particularly, to a wireless communication system for performing wireless communication such as by W-CDMA (Wideband-Code Division Multiple Access).
2. Description of the Related Art
W-CDMA is one of the wireless communication interfaces determined by IMT-2000 (International Mobile Telecommunications-2000) and is currently the leading wireless communication scheme. W-CDMA enables the access to multimedia such as sound, video and data at a maximum transmission rate of 384 Kbps.
Also, in recent years, wireless communication schemes called HSDPA (High Speed Downlink Packet Access) and HSUPA (High Speed Uplink Packet Access), which are based on W-CDMA technology, have been actively researched and developed. HSDPA and HSUPA allow, in existing W-CDMA systems, packets to be transmitted at high speeds in the downlink and uplink directions, respectively. HSDPA is standardized by 3GPP (3rd Generation Partnership Project) Release 5, and HSUPA by 3GPP Release 6.
FIG. 15 is a conceptual diagram illustrating W-CDMA communication. A base station 50 and terminals (UEs: User Equipments) 60-1 to 60-n communicate with each other in accordance with conventional W-CDMA.
On the uplink (user equipments 60-1 to 60-n→base station 50), DPDCH (Dedicated Physical Data CHannel), which is user information, and DPCCH (Dedicated Physical Control CHannel), which is control information, are transmitted to the base station 50 while being mapped respectively onto the in-phase component (I axis) and quadrature component (Q axis) of QPSK (Quadrature Phase Shift Keying) modulation.
On the other hand, on the downlink (base station 50→user equipments 60-1 to 60-n), DPDCH and DPCCH are transmitted to the user equipments 60-1 to 60-n by time-division multiplexing. These links are dedicated channels allocated to individual user equipments for the purpose of communication exclusively with the intended user equipments.
FIG. 16 is a conceptual diagram illustrating HSDPA communication. Downlink information is transmitted from the base station 50 to the user equipments 60-1 to 60-n by HSDPA. The user equipments 60-1 to 60-n individually receive a pilot signal f from the base station 50, then measure their respective propagation environments (received field strengths), and notify the base station 50 of the measurement results by HS-DPCCH (High Speed Dedicated Physical Control CHannel).
Based on the propagation environment information received from the user equipments 60-1 to 60-n, the base station 50 carries out scheduling to preferentially select a predetermined number of user equipments that are situated in good propagation environments.
If, as a result of the scheduling, the user equipments 60-1 and 60-2 are selected, for example, scheduling information (information including the type of modulation, the amount of transmission, etc.) is transmitted to the user equipments 60-1 and 60-2 by HS-SCCH (High Speed Shared Control CHannel). On receiving the scheduling information, the user equipments 60-1 and 60-2 set their respective functions in accordance with the received information.
Subsequently, the base station 50 transmits user information to the user equipments 60-1 and 60-2 via a radio channel called HS-PDSCH (High Speed Physical Downlink Shared CHannel). HS-PDSCH carrying the user information is a shared channel that can be used in common by the user equipments 60-1 to 60-n with one time slot shared by one or more user equipments, and permits high-speed downlink access at a maximum rate of 14.4 Mbps.
FIG. 17 is a conceptual diagram illustrating HSUPA communication. Uplink information is transmitted from the user equipments 60-1 to 60-n to the base station 50 by HSUPA. The user equipments 60-1 to 60-n transmit REQ (Request), as an uplink data transmission request, to the base station 50.
The base station 50 collects REQs from the user equipments 60-1 to 60-n, then carries out scheduling to determine the timings for uplink transmissions from the user equipments on the basis of the communication qualities of the user equipments 60-1 to 60-n, the priority levels of uplink data, etc., and transmits Grant, as permission of uplink transmission, to the user equipments 60-1 to 60-n (“Grant” includes two types of grant, namely, “absolute grant” whereby an uplink transmission rate and the like are notified at regular intervals, and “relative grant” whereby update of the information carried by “absolute grant” is notified).
The user equipments 60-1 to 60-n transmit user information to the base station 50, in order of permission of uplink transmission based on the Grants, by using dedicated channels called E-DCH (Enhanced Dedicated CHannel) which permits high-speed uplink access (attempts are currently made to increase the transmission rate of E-DCH up to about 2 to 4 Mbps).
Much expectation is placed on HSDPA/HSUPA as techniques that enable higher-speed data transmission than conventional W-CDMA. For HSDPA, the system specification is being formulated, but with respect to HSUPA, the specification for implementing the system is currently still in an investigation stage.
As conventional HSDPA/HSUPA techniques, a technique has been proposed in which data including a frame size corresponding with the scheduling interval is received over a signaling interface and the received data is transmitted within the scheduling interval (e.g., US 2004/0196870 A1 (paragraph nos. [0024] to [0030], FIG. 2)).
As a user equipment travels between cells, the transfer of service from the currently serving cell to another, namely, handover, takes place. In W-CDMA, an equivalent process called diversity handover is executed.
Diversity handover is characterized in that a user equipment being handed over simultaneously connects with multiple cells, and permits handover to be effected without momentary interruption and thus without influencing a stream being transmitted, such as sound, video or data (diversity handover is also called soft handover).
Also in the HSUPA described above, diversity handover may be executed, but in this case, multiple base stations to which a user equipment is connected perform HSUPA scheduling independently of one another, whereas the user equipment can use the scheduling information provided by only one of the multiple base stations. Accordingly, for the base stations whose scheduling information is not used, uplink transmission is executed at undesired timing, giving rise to an interference problem.
FIGS. 18 and 19 illustrate the problem caused at the time of diversity handover in HSUPA. Base stations 51 to 53 have cells 51a to 53a, respectively, and a user equipment (UE) 61 is connected to the base stations 51 to 53 to carry out diversity handover. Let us consider the case where the user equipment 61 transmits uplink data by HSUPA at the time of diversity handover.
The user equipment 61 transmits an uplink data transmission request to the base stations 51 to 53. The base stations 51 to 53 individually collect uplink data transmission requests from the user equipments (including the user equipment 61 and others) situated within their respective cells and carry out scheduling independently of one another.
Consequently, as shown in FIG. 18, the base station 51 generates scheduling information permitting the user equipment 61 to transmit uplink data at timing T1, the base station 52 generates scheduling information permitting the user equipment 61 to transmit uplink data at timing T2, and the base station 53 generates scheduling information permitting the user equipment 61 to transmit uplink data at timing T3. Each of the base stations 51 to 53 transmits the generated scheduling information to the user equipment 61, so that the user equipment 61 receives the three items of scheduling information.
The user equipment 61 can use only one of the three timings T1 to T3, and it is assumed here that the timing T1 is selected, as shown in FIG. 19, for the transmission of uplink data to the base stations 51 to 53.
In this case, uplink data is transmitted at the timing T1, and therefore, the base stations 52 and 53 receive no data at the respective timings T2 and T3. This means that the base stations 52 and 53 uselessly performed their scheduling processes.
Also, the scheduling of the base station 52 permits the transmission of uplink data at the timing T2, and this means that the transmission of uplink data at the other timing is forbidden. Accordingly, the uplink data output from the user equipment 61 at the timing T1 is a desired wave to the base station 51 but is an undesired interference wave to the base station 52.
Similarly, the base station 53 permitted the transmission of uplink data at the timing T3, and therefore, the uplink data transmitted at the timing T1 from the user equipment 61 is nothing but an undesired interference wave to the base station 53. Such an interference wave significantly lowers communication quality.